Maturing of Flat Panel Display (FPD) technologies has provided larger and lower cost laptop monitors, small area/low power panels for cell phones and other portable devices, HDTV and widescreen formats for home television, and high reliability daylight readable displays for “glass cockpits” for aircraft.
Emerging technologies such as organic LEDs (OLED) promise to deliver higher quality emissive flat displays, allowing the removal of the backlight. When compared to LCDs, a thinner form-factor with almost perfect viewing angle and much faster response speed would be provided by OLEDs. Thus the intrinsic characteristics of OLEDs give visual and form factor advantages over LCDs.
A typical array structure of an active matrix organic light-emitting diode (AMOLED) is shown in FIG. 1. The display 100 includes an array of pixels 102 that are arranged in rows and columns. The pixels 102 are connected to the data line 106 via a select transistor 104. The transistor 104 is a thin film transistor (TFT). The data line 106 is driven by a current source 108. The overlap capacitance of the transistors 104 connected to data line 106 and the line capacitance of the data line 106 itself leads to a high parasitic capacitance.
The basic OLED structure for a given pixel 102 consists of a stack of thin organic layers between a transparent anode and a metallic cathode. The organic layers include a hole-injection layer, a hole-transport layer, an emissive layer, and an electron transport layer. When an appropriate voltage is applied to the structure the injected positive and negative charges combine in the emissive layer to product light. OLEDs are therefore self-emissive displays and thus do not require a backlight as is required by LCDs. Also the charge combination process causes very little time delay providing for a fast response time.
OLED displays are current-controlled display devices. LCDs, on the other hand, are voltage-controlled. Current programming provides the OLED with a current that is independent of the characteristics of any other components such as thin film transistors (TFT) or the OLED itself, and compensates for Vt shift, spatial mismatch, and OLED degradation. However, the parasitic capacitance contributed from the line and select transistors connected to the line results in a large settling time. The settling time is a function of the initial line voltage and threshold voltage of the drive TFT. Although, the settling time can be improved partially by pre-charging, the improvement is not sufficient for medium and large area displays.
The parasitic capacitance of the drive transistor and the data line to which it is connected is schematically shown in FIG. 2. In particular FIG. 2 schematically shows the equivalent circuit for a current programmed pixel 202, having a current source 203 and a transistor 204, during a programming cycle. Capacitance CP 210 and resistance RP 208 are the parasitic components while capacitance CS 206 is the capacitance of the storage capacitor. If CS 206<<CP 210 and RP 208 is small, the timing constant, or settling time, of the circuit shown in FIG. 2 is:
                    τ        ∝                  2          ⁢                                          ⁢                                    C              p                                                      i                *                β                                                                        (        1        )            
where β is the coefficient in current-voltage (I-V) characteristics of the transistor 204 given by Ids=β (Vgs−Vth)2. Here, Ids is the drain-source current, Vgs the gate-source voltage, and Vth the threshold voltage.
If the capacitance Cp 210 is a large capacitance, around 40 pf, and β is small for the transistor 204, which is fabricated with amorphous silicon (a-Si), τ is of the order of millisecond. However, the timing budget of the programming cycle is less than 100 μs for large area displays. Since the efficiency of the OLED has been increased, the amount of current required to achieve the maximum brightness is very small; therefore, τ, which is also a function of current, increases dramatically.
This parasitic capacitance thus contributes to a high settling time for current programmed pixels, limiting the timing budget of the programming cycle. This can cause considerable error due to imperfect settling. In order to remove this error, a simple and fast solution for driving the current programmed pixels that is suitable for applications in OLED displays is needed.
United States patent application No. 20040095297A1 to Libsch et al. describes a programming method in which the programming current is controlled by a current sensor. A schematic diagram of the circuit of FIG. 1 of Libsch et al. is shown in FIG. 3. During the programming cycle a current sensor 302 monitors the voltage across resistor R 304 through the feedback 308. The current sensor 302 controls the programming current. After the pixel settles, the current flowing through the resistor R 304 and the OLED 306 is the same as wanted current. Because of the use of feedback 308, this driving method has a fast settling time. However, the drawback of this circuit is that it has a high power consumption resulting from resistor R 304. The resistor R 304 should quite large such that the circuit is able to sense a low current level accurately. Therefore, the power dissipated in resistor R 304 is considerable. The other drawback of this circuit is that it suffers from mismatch. The spatial mismatch changes the value of resistor R 304 causing non-uniformity in the display. It also has the addition feedback 308.
U.S. Pat. No. 6,433,488 to Bu discloses an OLED driver circuit that implements a current comparator in a feedback loop. The circuit presented in FIG. 2 of Bu is schematically presented in FIG. 4. In a programming cycle, SCAN is high so the transistor T2 402 is off and the transistor T4 404 is on. Therefore, the current flows through the transistor T3 406, the OLED 408, and the transistor T1 410. A current comparator 412 defines the reference voltage 414 based on comparison result of the pixel current, via feedback line 416, and reference current 418. After the pixel settles, the pixel current 416 is the same as reference current 418. This circuit provides a fast settling time for the pixel because of the use of feedback. However, the circuit has a high power compensation because of the two transistors (T1 410 and T2 402) in the path of current during the driving cycle, further this method uses four transistors and extra feedback line 416.
Therefore there is a need for a circuit that improves the settling time of the current driven circuit that does not encounter the high power consumption of the known circuits.